Detailed: During a strong El Niño ocean temperatures can average 2°C – 3.5°C (4°F - 6°F) above normal between the date line and the west coast of South America (bottom left map). These areas of exceptionally warm waters coincide with the regions of above-average tropical rainfall. During La Niña temperatures average 1°C - 3°C (2°F - 6°F) below normal between the date line and the west coast of South America. This large region of below-average temperatures coincides with the area of well below-average tropical rainfall.

For both
El Niño and La Niña the tropical rainfall, wind, and air pressure
patterns over the equatorial Pacific Ocean are most strongly linked to
the
underlying sea-surface temperatures, and vice versa, during
December-April. During this period the El Niño and La Niña conditions
are typically
strongest, and have the strongest impacts on U.S. weather
patterns.

El Nino
and La Niña episodes typically last approximately 9-12 months. They
often begin to form during June-August, reach peak strength during
December-April, and then decay during May-July of the next year.
However, some prolonged episodes have lasted 2 years and even as long
as 3-4
years. While their periodicity can be quite irregular, El Niño
and La Niña occurs every 3-5 years on average.

The Southern Oscillation and its Link to the ENSO Cycle

The fluctuations in ocean temperatures during El Niño and La Niña are
accompanied by even larger-scale fluctuations in air pressure known as the Southern
Oscillation. The negative phase of the Southern Oscillation occurs during El Niño
episodes, and refers to the situation when abnormally high air pressure covers Indonesia
and the western tropical Pacific and abnormally low air pressure covers the eastern
tropical Pacific. In contrast, the positive phase of the Southern Oscillation occurs
during La Niña episodes, and refers to the situation when abnormally low air pressure
covers Indonesia and the western tropical Pacific and abnormally high air pressure covers
the eastern tropical Pacific. These opposite phases of the Southern Oscillation are shown
above.